Discovering a Frosty Tale of Plant Evolution

Biology professor David Tank joins international team to create massive time-scaled evolutionary tree for land plants, explore history of adaptations for cold climates

By Tara RobertsPhotos by Simon Uribe-Convers

It’s no surprise to scientists that plants have traits that help them survive in freezing conditions. But a new paper co-authored by University of Idaho professor David Tank reveals an unexpected story behind these traits.

Tank and an international team of researchers have assembled the largest-ever time-scaled evolutionary tree of land plants. Researchers can now use the 32,223-species tree to uncover the “tempo and mode” of plant trait evolution, says Tank, an assistant professor of biological sciences.

The paper exploring cold-weather adaptations – published in the preeminent science journal Nature – is the first among an expected string of analyses gathered from the new tree and plant trait databases compiled by the team.

The research team identified three traits that help plants survive the cold:

seasonal leaf-dropping, or deciduousness – like oaks and maples in the autumn,

retreating underground, or herbaceous habit – like tomatoes that overwinter as seeds or tulips as bulbs

and having narrower water-conducting pathways to prevent freezing-induced air bubbles that block water flow.

Scientists previously assumed these traits evolved in direct response to encroaching cold weather. Analyses of these traits in the context of the time-scaled evolutionary tree indicates that leaf-dropping was such an adaptation, but retreating underground and developing narrower pathways happened before plants entered colder climates. This makes them exaptations – traits that evolved because of one condition, but offer an advantage in another as well.

“We’ve been able to put together a narrative that ties these all together,” Tank says. “These are traits that people assumed evolved in response to temperature. We’ve been able to show that most often woody plants became herbaceous or developed narrower water-conducting pathways before occupying freezing climates, suggesting that these two traits likely evolved in response to other factors.”

Tank’s primary contribution to the project was constructing an accurate and detailed evolutionary tree.

“This is a hard thing to do at this scale,” he says. “We had to create some novel tools to be able to do it.”

The researchers compiled existing information on seven gene regions commonly used to study evolutionary relationships in land plants. Tank combined computer analyses of these genes with existing knowledge of basic evolutionary relationships among plants to construct the tree’s framework.

Standard methods of creating time-scaled evolutionary trees are insufficient for a 32,000-species set, so Tank and UI postdoctoral researcher Jonathan Eastman developed an approach they call “congruification.”

“It takes a robust estimate of the timing of evolutionary branching points from a smaller, more tractable dataset and allows us to match these divergence points on the big tree so we can then estimate the timing of evolutionary splits at a much larger scale,” Tank explains.

To do this, Tank and Eastman used data from Tank’s work with the Angiosperm Tree of Life project, as well as dating information from fossils that have been used in multiple other studies. They also created a database of plant taxonomy, or classification, that is the most comprehensive to date.

In addition to the paper published in Nature, Tank and the research team expect to publish more analyses based on their data, including a study of what factors drove plants to develop the traits previously attributed to freezing climates.

They hope other scientists will use their research for their own analyses as well.

“One of the great things that we’ve done here is that all of the resources that we compiled for these analyses, including the tree and taxonomic resources, are publically available,” Tank says. “The working group has put together a large plant trait database, so with the phylogenetic context that we’ve provided, there are really a lot of questions you could explore at this scale.”

Interested scientists and non-scientists will have the chance to explore the evolutionary tree using the OneZoom Tree of Life Explorer – a web-based platform developed by former UI postdoctoral researcher James Rosindell with support from Luke Harmon, an associate professor of biological sciences at UI.